BRUSHLESS SERVOMOTORS BASIC INFORMATION

A synchronous machine with permanent
magnets on the rotor is the heart of the modern brushless servomotor
drive. The motor stays in synchronism with the frequency of supply,
though there is a limit to the maximum torque which can be developed
before the rotor is forced out of synchronism, pull-out torque being
typically between 1.5 and 4 times the continuously rated torque.

The torque–speed curve is therefore
simply a vertical line. The industrial application of brushless
servomotors has grown significantly for the following reasons:

● reduction of price of power
conversion products

● establishment of advanced control
of PWM inverters

● development of new, more powerful
and easier to use permanent magnet materials

● the developing need for highly
accurate position control

● the manufacture of all these
components in a very compact form

They are, in principle, easy to control
because the torque is generated in proportion to the current. In
addition, they have high efficiency, and high dynamic responses can
be achieved.

Brushless servomotors are often called
brushless dc servomotors because their structure is different from
that of dc servomotors. They rectify current by means of transistor
switching within the associated drive or amplifier, instead of a
commutator as used in dc servomotors.

Confusingly, they are also called ac
servomotors because brushless servomotors of the synchronous type
(with a permanent magnet rotor) detect the position of the rotational
magnetic field to control the three-phase current of the armature. It
is now widely recognized that brushless ac refers to a motor with a
sinusoidal stator winding distribution which is designed for use on a
sinusoidal or PWM inverter supply voltage.

Brushless dc refers to a motor with a
trapezoidal stator winding distribution which is designed for use on
a square wave or block commutation inverter supply voltage.

The brushless servomotor lacks the
commutator of the dc motor, and has a device (the drive, sometimes
referred to as the amplifier) for making the current flow according
to the rotor position. In the dc motor, increasing the number of
commutator segments reduces torque variation.

In the brushless motor, torque
variation is reduced by making the coil three-phase and, in the
steady state, by controlling the current of each phase into a sine
wave.